The term stereotaxis describes a technique, most often applied to the nervous system, in which the contents of the patient's skull (or body) are considered in a precise three dimensional space defined by a measuring instrument, such as a stereotactic frame, which is fixed to the patient's skull or body. Stereotactic frames are mechanical devices typically based upon a Cartesian or polar coordinate system. These systems typically include a means for securing the stereotactic frame device to the patient, at least one measuring scale for determining and confirming target coordinates and probe trajectories, and a probe holder/carrier.
The probe holder/carrier directs a surgical probe or some other instrument to a desired three dimensional location within the work space which is defined with respect to the geometry of the stereotactic frame. In practice, the stereotactic frame is also used to position a probe or other instrument inside the body into an anatomic or pathologic structure. The frame coordinates of the target structure are determined from stereotactic imaging studies including Computed Tomography (CT), Magnetic Resonance Imaging (MRI), ultrasonography, etc., and radiographically based procedures such as positive contrast ventriculography, or stereotactic atlases.
The use of stereotactic methods in the management of human brain tumors was first proposed in the early 1900's by the British physiologist, Robert Henry Clarke. Clarke patented a device for human stereotactic neurosurgery in 1912. However, the first human stereotactic procedure was not performed until 1947 when Spiegel and Wycis of Philadelphia attempted a ventriculography based dorsal median thalamotomy for psychiatric disease. Stereotactic instruments, methods and indications rapidly evolved thereafter. The three dimensional locations of intracranial targets were determined by means of stereotactic radiographically based methods, most commonly positive contrast ventriculography and stereotactic atlases based on the identification of radiographically established intracranial landmarks. The most common clinical use of stereotactic instrumentation in the late 1950's and 1960's was the placement of subcortical lesions to treat movement disorders, primarily the tremor of Parkinson's disease. However, following the introduction of L-Dopa in 1968 indications for stereotaxis decreased and the number of stereotactic procedures declined precipitously.
Nevertheless, old concepts in stereotactic frame design which were influenced by radiographically based point-in-space procedures carried over into the next phase of evolutionary stereotaxis. The advent of CT scanning in the early 1970's, and later magnetic resonance imaging, MRI scanning, rekindled interest in stereotactic surgery for two reasons: First, Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) provided a precise three dimensional data base which could be incorporated into the three dimensional coordinate system of a stereotactic frame. Secondly, in contrast to radiographically based examinations, one could actually see the complete intracranial tumors on these new computer based imaging modalities.
A progressive and expanding interest in CT and MRI based stereotactic procedures for intracranial tumors and other indications has been noted in neurosurgery since late 1979. Old stereotactic frames were modified to provide CT and MRI compatibility. New imaging compatible stereotactic frames were developed and introduced into the medical market. Nonetheless, the old concept of the stereotactic frame as a rigid device fixed to the patient's skull which was used to mechanically direct probes and other instruments to defined intracranial target points, persisted.
However, in the contemporary era of CT-and MRI-based stereotaxis, the coordinates for the intracranial target are derived from stereotactic CT and MRI examinations instead of a data base consisting only of projection radiographs. But during the stereotactic CT and MRI examinations the patient's head must still be fixed in the rigid, confining, stereotactic frame. CT and MRI opaque external fiducial reference marker systems are frequently applied to the frame to facilitate and simplify the calculation of stereotactic coordinates from the imaging modalities.
The mechanical constraints of a rigid stereotactic frame and the complexity of the computer-based imaging data bases act to limit most contemporary stereotactic procedures to point-in-space targets for biopsy, placing interstitial catheters, localizing a bone flap over a tumor and the like. However, the availability of high capacity and low cost computer work stations have provided a method for reducing the complexity of the image reconstructions and target point cross registration between imaging modalities.
In addition, computer interactive stereotactic methods allow tumors defined by CT and MRI to be considered as volumes in space and provide to surgeons graphical displays which indicate the CT and MRI defined boundaries of the lesion within a defined stereotactic surgical field. This volumetric display technique is superior because it is necessary to remove the entire volume of the tumor, especially at its outer boundary, to ensure that further growth does not take place after the operation.
Such technique for volumetric stereotaxis was first proposed by Kelly et al in 1982. In these procedures, a tumor volume is reconstructed from stereotactic CT or MRI data and reformatted along a surgical viewing trajectory defined by a stereotactic frame. During surgery, an operating room computer system displays cross sections of the reformatted tumor volume with respect to surgical instruments directed into the surgical field using the stereotactic frame as a reference source. Intra-operatively, the surgeon monitors the computer generated image of the surgical field which was derived from CT or MRI scans, as well as the surgical field itself.
In 1986 a system was developed for superimposition of the computer image upon the surgical field by means of a heads-up display unit attached to the operating microscope. This allowed the surgeon to simultaneously view updated reformatted and scaled images of the CT or MRI defined surgical field visually superimposed upon the actual surgical field. Since 1984 over 2300 computer-assisted stereotactic procedures and more than 800 computer-assisted volumetric stereotactic tumors resections have been performed at the Mayo Clinic. It has been found that these procedures allow a more complete removal of a tumor in a minimally invasive way.
However, stereotactic systems to date have had a series of associated problems. Stereotactic frames are cumbersome in general. They are especially cumbersome for procedures requiring more than a few target points and in volumetric stereotactic procedures where the demands of the procedure dictate the need for a larger working area, yet where such demand comes into conflict with the physical structure of the frame. A conventional stereotactic frame is typically a cage structure extending about the patient's head and therefore inherently restricts the freedom of movement of the surgeon. Changing a target point or trajectory to reach that new target point involves a separate mechanical adjustment of the stereotactic frame. This is not usually a problem in point-in-space stereotaxis, as in biopsy procedures for example.
But many mechanical adjustments become very cumbersome when a surgeon is confronted with an infinite number of points which define the boundary of a volumetric lesion. In addition, a stereotactic reference frame must be applied to a patient's head in order to acquire a pre-surgical data base.
Some surgeons find the stereotactic frame application procedure difficult and time consuming. Patients also find this uncomfortable. In addition, the necessity to repeat CT and MRI examinations for the pre-surgical data base increases the cost to the patient. Finally many surgeons are intimidated by mechanically complex devices in general and stereotactic frames in particular. Furthermore, although the mathematics in stereotaxis are understandable, many surgeons are uncomfortable with these also.
Since 1987 there has been an interest in so-called frameless stereotactic procedures. In the procedures described so far, a multi-jointed digitizing arm is indexed to the patient's head. Typically, precision potentiometers or optical encoders on each of the joints of the digitizing arm provide feedback from which real world coordinates of a three dimensional point are determined by a host computer system.
In some systems, reference marks are placed on the patient's scalp. Imaging studies are performed at surgery. The surgeon uses the digitizing arm to touch these registration points. The coordinates of these known points correspond to reference marks on the imaging studies. The computer can then calculate a transformation matrix to allow transformation of the real world coordinate system to the coordinate system of the imaging study. In practice, a cursor, which corresponds to the position of the tip of the pointer in the surgical field, is displayed on CT or MRI slices or three dimensionally rendered images based on CT or MRI.
The problem with these multi-jointed digitizing devices is that they also are cumbersome and restrict the surgeon's freedom of movement. In addition, the 5 or 6 encoders at each of the 5 or 6 joints in the multi-jointed digitizing device can occasionally combine to form a significant non-offsetting error resulting in unpredictable results in surgery.
Although advances in technology include faster hardware and software, improved error detection and improved mathematical treatment in the techniques of producing the images produced and better resolution with respect to individual images selected from an object to be scanned, few such improvements have been directed toward the surgical practitioner to facilitate his working through the procedure. Such improvements are needed to balance the improvements in hardware and software to improve the overall effectiveness of the surgeon's skill.
Several advances in the software and hardware areas have been made, however none enable the effective use and manipulation of two dimensional imaging system as keyed to a three dimensional locational system to be used with a patient during surgery. The following patents outline some of these improvements.
U.S. Pat. No. 4,849,692 issued on Jul. 18, 1989 to Ernest B. Blood and entitled "Device for Quantitatively Measuring the Relative Position and Orientation of Two Bodies in the Presence of Metals Utilizing Direct Current Magnetic Fields" uses two or more transmitting antenna of known position and orientation. Each transmitting antenna is driven, one at a time by a pulsed direct current signal. The receiving antenna measure the transmitted signals, one axis at a time, and then measures the earth's magnetic signal, one axis at a time.
U.S. Pat. No. 4,945,305, issued on Jul. 31, 1990 to Ernest B. Blood and also entitled "Device for Quantitatively Measuring the Relative Position and Orientation of Two Bodies in the Presence of Metals Utilizing Direct Current Magnetic Fields" describes improvements to the '692 patent and improved locational and orientational data. Both the U.S. Pat. Nos. 4,849,692 and 4,945,305 patents relate to the use of magnetic fields to determine location in a three dimensional area despite the occasional and changing presence of metallic bodies which usually serve to distort the very field which is being relied upon for measurement. Both the U.S. Pat. Nos. 4,849,692 and 4,945,305 relate to locating a point in three dimensional space and are unrelated to either imaging or surgery.
U.S. Pat. No. 4,951,653 to Fry et al entitled "Ultrasonic Brain Lesioning System" discloses the use of ultrasound, CT (computerized axial tomography), or MRI (magnetic resonance imaging) in probing for site localization in conjunction with a skull fixation system. A precision ball provides linear and rotary positioning data by way of a cup fitting over a plurality of spheres and a linear encoder which interfaces with the cups. A bulky apparatus is utilized with a series of joystick type "precision balls" to direct an ultrasound signal through a cooling media to produce volume lesions in the brain at the site of identified brain tumors. The device is not utilized with open surgery, and the production of lesions is based upon the machine targeting of pre-existing tumors and the production of lesions without cutting the body.
U.S. Pat. No. 4,959,610 to Suzuki et al, entitled "Magnetic Resonance Apparatus" discloses details pertinent to NMR electromagnetic field atomic theory. U.S. Pat. No. 5,050,608 to Watanabe et al, entitled "System for indicating a Position to be Operated in a Patient's Body," discloses the use of an articulated probe as a control source for displaying a series of tomographical images on a cathode ray tube. This device assumes the orientation of the patient with respect to the device. If the patient moves, especially during surgery, the mechanical pointer may be pointing to a portion of the patient's anatomy such that the computer controls produce a tomographical image which is keyed to another portion of the patient's anatomy.
Further, since the Watanabe device includes four joints, a significant error is introduced in the resolution since the spatial Location of the pointer is dependent on the positions of the various angular displacement sensors at the joints of the articulated arm of the pointer. Further, the pointer is made collapsible so that it can be pushed toward the scalp to indicate an affected part in order for a surgeon to be enabled to make an incision. A solid tip can be pushed into the brain to indicate the depth of the tip into the brain while using the notches on the pointer to measure depth into the brain. This device is stated as being useful to assist in the initiation of surgery rather than assist during surgery, particularly since there is no provision to account for movement of the patient during the surgery.
U.S. Pat. No. 5,094,241 to Allen, entitled "Apparatus for Imaging the Anatomy," involves the placement of implants below the skin level and on the bones in order to key a patient to an imaging system. This method is utilized to account for problems in re-imaging for circumstances where significant amounts of time pass between a first and subsequent examinations. In such cases, a shift in viewing angle which might make a volume of interest appear larger or smaller compared to a subsequent examination, is corrected for by using the implants. The implants utilized must be of the type which will show up on a tomographic imaging system in order to register it to the images produced in a given examination.
U.S. Pat. No. 5,107,839 to Houdek et al and entitled "Computer Controlled Stereotaxic Radiotherapy System and Method," discloses a low frequency electromagnetic position detection means. The patient's head is repetitively position re located using a halo attached to the patient's head with skin piercing screws and which operates within a large multi-structured stereotaxic cage. The halo and cage would significantly interfere with surgery, and would be affected by the presence of metal surgical tools within the stereotaxic cage.
U.S. Pat. No. 4,791,934 to Brunnett, entitled "Computer Tomography Assisted Stereotactic Surgery System and Method" discloses a system using a multi-jointed referencing system. In the Brunette patent, a CT scan occurs at one location and is digitally stored in a computer. At a second location the patient is positioned in a digital radiographic imaging device utilized to produce a shadowgrahic image which is also stored. The shadowgraphic image is then registered with the scan image which may be accomplished visually on a video monitor on an operating table and "registered" in 3 dimensions with the CT data using shadowgraphic images shown on a display. The Brunette patent teaches that once the patient is "registered" the surgeon can then plan the best path of entry with a biopsy needle. The shadowgraphic image is formed with an X-ray device, thus presenting concerns about excess radioactive exposure of the patient.
Most of these systems are bulky, cumbersome, and generally more related to matching the coordinates of one system with those of another system. Each one presents an advancement in the art, but all fail to teach the construction of a system which joins the coordinate resolution of a physical three dimensional system with the two dimensional nature of non-invasive imaging.